Aerial work platforms have been developed and in use for more than twenty five years. Their primary purpose is to raise workers quickly and safely to positions to do necessary work. They replace earlier means of access such as ladders and scaffolds. Some models can also reach below a surface or a long horizontal distance from the surface on which they are located, and some types of lifts can place workers as high as 150 to 250 feet above the ground.
The first three of the following make up the major population of such machines:
1. Vehicle-Mounted Elevating and Rotating Aerial Devices as covered by ANSI/SIA A92.2-1990. These machines are typically mounted on a commercial truck chassis but may be mounted on a trailer chassis and are used in erection and maintenance of utility lines. They include models where the work platform is supported by an articulating or a telescoping boom which is mounted on a turntable that can rotate about a vertical axis.
2. Boom-Supported Elevating Work Platforms as defined in ANSI/SIA A92.5-1992. These machines are self-propelled, typically have a telescoping and/or articulating boom and are used in construction and maintenance tasks.
3. Self-Propelled Elevating Work Platforms as defined in ANSI/SIA A92.6-1990. These machines (typically scissor-lifts) elevate the work platform vertically but do not position the platform horizontally completely outside the base on which it is supported. They are also used in general construction and maintenance tasks.
4. Manually Propelled Elevating Work Platforms as defined in ANSI/SIA A92.3-1990. These machines are manually propelled and have platforms that cannot be positioned completely outside the base.
5. Airline Ground Support Vehicle-Mounted Vertical Lift Devices as defined in ANSI/SIA A92.7-1990. These are machines designed specifically for aircraft servicing and maintenance.
6. Vehicle-Mounted Bridge Inspection and Maintenance Devices as defined in ANSI/SIA A92.8-1993. These machines are designed to reach out, down and under a bridge for inspection and maintenance.
7. Mast-Climbing Work Platforms as defined in ANSI/SIA A92.9. These machines are designed to place several workers on a platform along a wall or similar vertical surface to do extensive operations.
Controls for operating most of the types of aerial platforms mentioned above are comprised of electrical switches or other devices mounted at an operator's station on the platform. These electrical devices control valves or other means on the chassis which in turn activate hydraulic or electrically powered devices such as cylinders or motors. Mechanical controls are difficult or impossible to use for controls on the platform because of the distance from the platform to the chassis and the mechanical positioning of the platform relative to the chassis in order to reach the desired work location. Likewise, it is difficult and inefficient, except in the simplest machines, to route multiple hydraulic lines with hoses at mechanical joints up to the platform where hydraulic valves could be used to control machine motions. Therefore, the industry practice has evolved to the use of electrical switches and controllers on the platform which actuate hydraulic or electrical means on the chassis to cause the desired motion. Two general types of electrical devices are (1) the simple on/off switch that may be actuated in two directions, e.g., up or down, and usually is spring loaded to return to neutral, and (2) the controller type of switch that usually provides an electrical output signal proportional to the displacement of the handle of the controller. The proportional controller is important to the operator and is used to make smooth starts and stops and to move at a reduced rate of speed as existing conditions may make desirable. Proportional controllers are used on most sophisticated aerial work platforms and on those providing greater platform height. The two types are used interchangeably in this document and it will be understood that the word "switch" is to be interpreted broadly and includes a controller, and vice versa.
Switches were typically located on the operator's control panel in patterns which may have been influenced by aesthetics, space considerations and fabrication economy. Beginning in 1980, the applicable consensus standard for boom-supported elevating platforms (ANSI A92.5-1980) specified that "all directional controls shall move in the direction of the function which they control when possible, and shall be of the type which return to the `off` or the neutral position when released. Such controls shall be protected against inadvertent operation." Directional controls are defined in the ANSI A92.5-1980 Standard as "all controls necessary to raise, lower, rotate, telescope, drive or otherwise initiate the powered functions of the work platform." A similar requirement had first appeared in the ANSI A92.6-1979 Standard on Self-propelled Elevating Work Platforms, albeit without the definition of "directional controls."
One design with clear advantages with respect to earlier control arrangements is disclosed and described in U.S. Pat. No. 4,331,215--Grove et al. This patent discloses controls that are individual electric devices mounted on a surface slightly inclined to the vertical or on a second surface slightly inclined to the horizontal. This arrangement permits all of the controls to operate in substantially the same direction in which the platform moves as a result of the control activation. This design meets the requirement of the applicable consensus standard (ANSI A92.5-1980) and provides an approach that minimizes operator error, a major cause of accidents on aerial work platforms.
As noted in the Grove et al. patent workers such as electricians, painters, sandblasting operators, bricklayers and carpenters using these machines are skilled primarily in the area of their work specialty and the aerial platform they are using serves solely as a positioning means, hence, many operators do not become proficient as do the trained operators of cranes or earth-moving machinery who do nothing but operate such machines full time. Moreover, operators of aerial platforms may use one machine for a few days and may then be assigned a different make and model that has a different control arrangement, or may even rent different machines for use on an "as needed" basis. Although operators of aerial platforms are required to be trained, such training may be limited and often does not include specific training on the control variations used on different makes and models. Therefore, the opportunity for inadvertent errors is increased by the requirement for the operator to first select the proper control, second, to check to be certain of which way to operate the control handle, and third, to then implement the control operation.
Other machines utilize controls which differ from the typical steering wheel, accelerator, brake, and gear shift with which most people are familiar. Various construction vehicles such as skid-steer loaders, bulldozers, and front end loaders are provided with control levers usually based on the mechanical devices which effect the motion but do not necessarily move in a direction of the motion caused nor do they provide a simulated model of the machine for quick operator recognition and orientation. Certain models of power lawn mowers currently available also utilize levers to effect driving and steering but lack the analogous motion and the simulated model of the machine for quick operator recognition and orientation. Another control layout which was developed on early aircraft and is still used on certain light aircraft and military fighters is the pilot's cockpit control stick. When the control stick is pushed to the left, the aircraft rolls to the left in response; when the stick is pushed forward, the aircraft rotates nose downward in response. However, the control stick does not provide the third axis control for the rudder, and, most importantly, does not provide the simulated model which ensures the quick recognition and orientation needed for aerial platform operators. Pilots are required to be trained and licensed, even for light civilian aircraft, while military aircraft can only be flown by pilots who have hundreds of hours of training and flight experience. On the other hand, training of aerial platform operators is often minimal; indeed, a worker at a large construction site will often come upon an aerial platform not in use, will start it if possible, and will proceed to use it without permission or any training.
Further requirements for the operator's controls are specified in the ANSI/SIA A92.6-1990 Standard for Self-Propelled Elevating Work Platforms that require that the upper controls (on the platform) shall "include a control which shall be continuously activated in order for upper directional controls to be operational and which automatically returns to the off position when released." A similar requirement is specified in the ANSI/SIA A92.5-1992 Standard for Boom-Supported Elevating Work Platforms by specifying that the upper controls provided at the platform shall "include a separate safety control which shall be continuously activated for upper directional controls to be operational, and which renders upper directional controls inoperative when released." These requirements have typically been met by having a separate foot pedal or an equivalent switch that must be operated by the operator in order for the directional controls to be used. A foot pedal can be actuated by the operator while using one hand to operate the directional control. However, in addition to the cost and installation expense, the foot pedal requires an electrical cord connection which must be durable, and together with the pedal is subject to rough service, deterioration due to weather conditions and damage from falling objects. A separate hand switch may be utilized but the operator may then be required to use both hands. A safety control may be integrated into each directional control but this would require three or more duplicate safety switches, with the controls of each capable of being released by the operator if he or his hand is trapped such that he cannot return the directional control to neutral. The safety control, in order to be most effective and provide maximum safety enhancement, must meet at least the following requirements:
1. It must prevent inadvertent operation of a directional control in case the directional control is struck by the operator, by other personnel or by a falling object or tool being used in performance of the work task;
2. When released it would preferably provide a signal to stop all powered functions if a malfunction occurs in the directional control or any other component of the control system;
3. When released it would preferably provide a signal to stop all powered functions if a malfunction occurs in any component of the power supply system; and
4. It would preferably provide a signal to stop or prevent unsafe powered motion that may be caused by a single point failure mode.
Thus, despite the advances made in the art, the control systems and arrangements discussed above are deficient in not providing a total human factors solution and in not utilizing a control configuration and mechanism that can provide rapid and certain recognition and orientation to both trained and untrained operators, resulting in increased safety and operator efficiency.